A Model–Data Comparison for a Multi-Model Ensemble of Early Eocene Atmosphere–Ocean Simulations: Eomip

Clim. Past, 8, 1717–1736, 2012 www.clim-past.net/8/1717/2012/ Climate doi:10.5194/cp-8-1717-2012 of the Past © Author(s) 2012. CC Attribution 3.0 License. A model–data comparison for a multi-model ensemble of early Eocene atmosphere–ocean simulations: EoMIP D. J. Lunt1, T. Dunkley Jones2,*, M. Heinemann3, M. Huber4, A. LeGrande5, A. Winguth6, C. Loptson1, J. Marotzke7, C. D. Roberts8, J. Tindall9, P. Valdes1, and C. Winguth6 1School of Geographical Sciences, University of Bristol, UK 2Department of Earth Science and Engineering, Imperial College London, UK 3International Pacific Research Center, University of Hawaii, USA 4Department of Earth and Atmospheric Sciences, Purdue University, USA 5NASA/Goddard Institute for Space Studies, USA 6Department of Earth and Environmental Science, University of Texas at Arlington, USA 7Max Planck Institute for Meteorology, Hamburg, Germany 8The Met Office, Exeter, UK 9School of Earth and Environment, University of Leeds, UK *now at: School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK Correspondence to: D. J. Lunt ([email protected]) Received: 29 March 2012 – Published in Clim. Past Discuss.: 16 April 2012 Revised: 19 September 2012 – Accepted: 20 September 2012 – Published: 29 October 2012 Abstract. The early Eocene (∼ 55 to 50 Ma) is a time pe- of atmospheric CO2 and temperature during this time period which has been explored in a large number of mod- riod will allow a quantitative assessment of the skill of the elling and data studies. Here, using an ensemble of previ- models at simulating warm climates. If it is the case that a ously published model results, making up “EoMIP” – the model which gives a good simulation of the Eocene will also Eocene Modelling Intercomparison Project – and synthe- give a good simulation of the future, then such an assessment ses of early Eocene terrestrial and sea surface temperature could be used to produce metrics for weighting future cli- data, we present a self-consistent inter-model and model– mate predictions. data comparison. This shows that the previous modelling studies exhibit a very wide inter-model variability, but that at high CO2, there is good agreement between models and data for this period, particularly if possible seasonal biases 1 Introduction in some of the proxies are considered. An energy balance analysis explores the reasons for the differences between the Making robust predictions of future climate change is a ma- model results, and suggests that differences in surface albedo jor challenge, which has environmental, societal, and eco- feedbacks, water vapour and lapse rate feedbacks, and pre- nomic relevance. The numerical models which are used to scribed aerosol loading are the dominant cause for the dif- make these predictions are normally tested over time peri- ferent results seen in the models, rather than inconsisten- ods for which there are extensive instrumental records of cies in other prescribed boundary conditions or differences climate available, typically over the last ∼ 100 yr (Hegerl et al., 2007). However, the variations in climate over these in cloud feedbacks. The CO2 level which would give optimal early Eocene model–data agreement, based on those models timescales are small relative to the variations predicted for the next 100 yr or more (Meehl et al., 2007), and so likely which have carried out simulations with more than one CO2 level, is in the range of 2500 ppmv to 6500 ppmv. Given the provide only a weak constraint on future predictions. As spread of model results, tighter bounds on proxy estimates such, proxy indicators of climate from older time periods are increasingly being used to test models. On the timescale of Published by Copernicus Publications on behalf of the European Geosciences Union. 1718 D. J. Lunt et al.: EoMIP ∼ 100 000 yr, the Paleoclimate Modelling Intercomparison temperature indicators, taking full account of uncertain- Project (PMIP, Braconnot et al., 2007), now in its third phase, ties in the reconstructions. is focusing on three main time periods: the mid-Holocene (6000 yr ago, 6 k), the Last Glacial Maximum (LGM, 21 k), – By analysing the energy balance and fluxes in the mod- and the Last Interglacial (LIG, 125 k). However, these time els, to gain an understanding of the reasons behind the periods are either colder than modern (LGM), or their differences in the model results. warmth is primarily caused not by enhanced greenhouse Section 2 describes the model simulations, Sect. 3 presents gases, but by orbital forcing (mid-Holocene, LIG). As such, the datasets used to evaluate the models, and Sect. 4 presents their use for testing models used for future climate prediction the model results and model–data comparison. Section 5 is also limited. On the timescale of millions of years, sev- quantifies the reasons for the differences between the model eral time periods show potential for model evaluation, being results, and Sect. 6 discusses, concludes, and proposes direc- characterised by substantial warmth which is thought to be tions for future research. driven primarily by enhanced atmospheric CO2 concentra- tions. An example is the mid-Pliocene (3 million years ago, 3 Ma), when global annual temperature was ∼ 3 ◦C greater 2 Model simulation descriptions than pre-industrial (Dowsett et al., 2009). However, the latest estimates of mid-Pliocene CO2 (Pagani et al., 2010; Seki Many model simulations have been carried out over the last et al., 2010) range from ∼ 360 to ∼ 420 ppmv, which is sim- two decades with the aim of representing the early Eocene. ilar to that of modern (∼ 390 ppmv in 2010 according to the Here, we present and discuss results from a selection of Scripps CO2 program, http://scrippsco2.ucsd.edu/), and sub- these. We present all simulations of which we are aware that stantially less than typical IPCC scenarios for CO2 concen- (a) are published in the peer-reviewed literature, and (b) are tration at the end of this century (∼ 1000 ppmv in the A1F1 carried out with fully dynamic atmosphere–ocean general scenario, > 1370 ppmv CO2-equivalent in the RCP8.5 sce- circulation models (GCMs), with primitive equation atmo- nario, Meehl et al., 2007; Moss et al., 2010). The time period spheres. This makes a total of 4 models – HadCM3L (Lunt which shows possibly the most similarity to projections of et al., 2010), ECHAM5/MPI-OM (Heinemann et al., 2009), the end of the 21st century and beyond is the early Eocene, CCSM3 (Winguth et al., 2010, 2012; Liu et al., 2009; Huber ∼ 55 to ∼ 50 Ma. A recent compilation of Cenozoic atmo- and Caballero, 2011), and GISS ModelE-R (Roberts et al., spheric CO2 is relatively data-sparse during the early Eocene, 2009). Criterion (b) is chosen to select the models which are with large uncertainty range, meaning that values more than most similar to those used in future climate change projection 2000 ppmv cannot be ruled out (Beerling and Royer, 2011). (i.e. we exclude models with energy balance atmospheres Relatively high values for the early Eocene are consistent such as GENIE; Panchuk et al., 2008). There are two sets with recent CO2 reconstructions for the latest Eocene of the of CCSM3 simulations, which we name CCSM W(Winguth order 1000 ppmv (Pearson et al., 2009; Pagani et al., 2011). et al., 2010, 2012) and CCSM H(Liu et al., 2009; Huber Proxy indicators have been interpreted as showing tropical and Caballero, 2011). All the models and simulations are temperatures at this time ∼5 ◦C warmer than modern (e.g. summarised in Table 1. Together they make up the “Eocene Pearson et al., 2001), and high latitude terrestrial tempera- Modelling Intercomparison Project”, EoMIP. EoMIP differs tures more than 20 ◦C warmer (e.g. Huber and Caballero, from more formal model intercomparisons, such as those 2011). Recently, due at least in part to interest associated carried out under the auspices of PMIP, in that the groups with this time period as a possible future analogue, there have carried out their own experimental design and simula- have been a number of new sea surface temperature (SST) tions in isolation, and the comparison is being carried out and terrestrial temperature data published, using a range of post hoc, rather than being planned from the outset. As such, proxy reconstruction methods. There have also been several the groups have used different paleogeographical boundary models recently configured for the early Eocene and attempts conditions and CO2 levels to simulate their Eocene climates. made to understand the mechanisms of Eocene warmth. Most This has advantages and disadvantages compared to the more of these studies have carried out some form of model–data formal approach with a single experimental design: The main comparison; however, the models have not been formally in- disadvantage is that a direct comparison between models is tercompared in a consistent framework, and new data now impossible due to even subtle differences in imposed bound- allows a more robust and extensive evaluation of the models. ary conditions; the main advantage is that in addition to un- The aims of this paper are: certainties in the models themselves, the model ensemble also represents the uncertainties in the paleoenvironmental – To present an intercomparison of five models, all re- conditions, and therefore more fully represents the uncer- cently used to simulate the early Eocene climate. tainty in our climatic predictions for that time period. – To carry out a consistent and comprehensive comparison of the model results with the latest proxy Clim. Past, 8, 1717–1736, 2012 www.clim-past.net/8/1717/2012/ D. J. Lunt et al.: EoMIP 1719 Table 1. Summary of model simulations in EoMIP. Some models have irregular grids in the atmosphere and/or ocean, or have spectral atmospheres.

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